Provided is a small, simple-structured, homogeneous and mass-producible excessive pressure release valve which works even at a relatively low excessive pressure to surely and safely release the pressure to the external world. The excessive pressure release valve has an elastic rubber plate and a reversibly openable slit that passes through the upper and the lower surfaces and release the pressure to the external world when the internal excessive pressure stressing on the lower side surface, pushes and opens the slit toward upper surface side. The elastic rubber plate is made of silicone rubber containing 1-20% by mass of silicone oil.